Automatic merge from submit-queue (batch tested with PRs 45610, 47628)
Add Priority to Kubernetes API
**What this PR does / why we need it**: This is the first in a series of PRs to add priority to Kubernetes API. Subsequent PRs will add priority name resolution to admission controller.
**Release note**:
```release-note
Add PriorityClassName and Priority fields to PodSpec.
```